1. Field of the Invention
The present invention relates to a fuel injection control device of a diesel engine equipped with an exhaust purification device.
2. Description of the Related Art
It is important for a diesel engine to decrease such things as particulate matter (hereinafter referred to as PM), nitrogen oxide (NOx) and hydrocarbon (HC) contained in an exhaust gas. Various types of exhaust purification devices have been proposed in order to decrease such substances.
For example, there is known a so-called continuation reproduction type DPF (diesel particulate filter) in which a catalyst such as zeolite is used with a monolith honeycomb filter of a wall flow type made from ceramics or a fiber type filter made from ceramics and metals. In the DPF, PM in the exhaust gas is collected by the filter, and the collected PM is burned (oxidized) and removed by catalytic action.
However, these exhaust purification devices with such continuation reproduction type DPF which utilize catalytic action do not provide satisfying results in the exhaust gas purification unless the exhaust gas temperature is at activation temperature (for example; 250 degrees C. or more) for the catalyst.
Accordingly, if the exhaust gas temperature is low e.g., when an engine has just started and a vehicle is running with a light load, there is a possibility that the exhaust gas would not be purified by the exhaust purification device.
When the exhaust gas temperature is below the activation temperature of the catalyst provided in an exhaust purification device, an engine intake throttle valve is narrowed to decrease the amount of air supplied to the engine, thereby increasing an air-fuel ratio to raise exhaust gas temperature. However, by doing this, the amount of exhaust gas itself is reduced. This means that the elevated exhaust gas temperature does not contribute to the exhaust gas purification very much.
Hence, it has been proposed to retard the fuel injection timing as an alternative method of raising the exhaust gas temperature. By doing so, exhaust gas temperature can be greatly raised.
However, when the fuel injection timing is retarded, there is a problem that engine torque output decreases.
This will be described in reference to FIG. 5 of the accompanying drawings.
In FIG. 5, the amount of fuel injection is indicated by the horizontal axis and the torque output of the engine is indicated by the vertical axis. The line A shows a torque output when the fuel injection timing is conducted at normal timing. The line B shows a torque output when the fuel injection timing is retarded for a predetermined period.
As understood from FIG. 5, when the same amount of the fuel injection is supplied, the torque output obtained when the fuel injection is retarded is smaller than the torque output obtained when the fuel injection is performed at the normal timing. For example, if the fuel injection timing is set slower from the state indicated by the point C in FIG. 5 in order to raise exhaust gas temperature, the torque output would decrease from P1 to P2, and this may cause a driver to feel a shock.